1. Field of the Invention
This invention relates to the preparation of high molecular weight polymers having a low residual unreacted monomer content by a gel-type polymerization process. Such polymers are frequently used as flocculants for the treatment of impure water, and may find other uses in drag reduction, adhesives, coatings, and textile and paper sizes.
2. Description of the Prior Art
A gel-type polymerization process can be used to manufacture polymers. That is, an aqueous solution containing between 20 and 60% by weight total monomer is subject to polymerization such that a stiff, rubbery gel is formed from the initially liquid solution. The gel is then pulverized to form particles about 4 mm in size, followed by drying to remove about 90% of the water present. Finally, the dried particles are crushed to a smaller size to produce a granular powder suitable for sale.
It has long been desired to reduce the residual monomer content of polymers produced by gel-type polymerization. Acrylamide monomer, for example, is deemed toxic, and governmental regulations place an upper limit on the permissible acrylamide monomer content. This limit generally varies from between 250 ppm to 1000 ppm depending on the intended end-use, which might include clarification of water ultimately intended for human consumption. New legislation in some European countries, however, may deem the upper acceptable limit to be as low as 100 ppm, which is difficult or expensive to achieve in certain types of polymeric products using known manufacturing techniques. Because of this legislation, as well as a general common-sense desire to reduce the toxicity of products that may be handled by people and used for an application such as treatment of drinking water, there exists a need for efficient manufacturing processes that can produce acrylamide polymers having a very low content (less than 100 ppm) of residual acrylamide.
Gel polymerizations on an industrial scale generally use a redox or an azo initiating system, often in combination with each other. In a redox initiation system polymerization is initiated by radicals formed by an oxidizing agent and a reducing agent used together, such as a combination of persulfate (oxidizing agent) and thiosulfate (reducing agent). In an azo initation system, radicals are produced thermally using one or more azo initiators, such as azobis(2-amidinopropane)dihydrochloride. As used herein, a redox-azo intiation system refers to a polymerization in which both redox and azo initiators are used.
Alternatively a photopolymerization process may be used, whereby one or more photoinitiators is added and the monomer solution irradiated by ultraviolet (UV) or visible light. A redox/azo system can also be combined with a photopolymerization process. For each of these gel polymerization systems there exist methods to achieve low residual monomer, but each has its own shortcomings.
The residual monomer present in ground gel produced from any of the aforementioned polymerization processes can be reduced by at least three general methods. One method is extractive washing which uses methanol or other solvent, as described, for example in Japanese Patent Publication JP-P 53-051289. However, this method requires the use of a large amount of flammable solvents and is not desirable from the viewpoint of safety and economy.
Another method for the reduction of residual monomer is by addition of an alkali metabisulfite or sulfite, or by treatment with sulfur dioxide, as described in U.S. Pat. Nos. 3,755,280, 4,929,717, and 4,306,955 and Japanese Patent Publication JP-P 61-115909. However these methods can result in a discharge of sulfur dioxide during the subsequent drying step, particularly in the case of gels made at a pH less than 4, as is the usual case when acrylamide is copolymerized with cationic ester monomers. Such discharge in the dryer results in an environmentally unacceptable emission to the atmosphere, necessitating expensive scrubbing equipment. Another problem with sulfite and sulfur dioxide treatments is that polymer degradation can occur, leading to decreased flocculant performance.
A third general method for reduction of residual monomer is by treatment with amidase enzyme, as described in published PCT Application Nos. WO 97/29136 and WO 99/07838. However, amidase enzyme and compositions containing amidase enzyme are relatively expensive, and the residues left in the product may be hazardous to people and fall under regulatory scrutiny. An additional drawback of amidase treatment is the greatly decreased effectiveness on cationic gels made at low pH, as well as a further reduction in effectiveness that occurs as a result of the common practice of including organic acids such as adipic acid in the cationic gels.
A general problem with the methods described in the above-cited patents (with the exception of the methods using sulfur dioxide gas, which poses health hazards and can be corrosive to metal equipment) is that the methods require a liquid or solid substance to be intimately mixed into the gel. This can require specialized equipment and is difficult to accomplish on an industrial scale.
With regard to a redox or redox/azo polymerization system, there are several ways to produce polymer with reduced residual monomer content. The polymerization time can be lengthened, optionally in conjunction with heating to hold the gel at a high temperature, as described in U.S. Pat. No. 4,132,844. However, this option leads to a decrease in the production rate on an industrial scale or the need for a large amount of plant space. Another method is to increase the amount of redox and/or azo initiator, but this generally leads to gelled polymers having lower molecular weight and decreased flocculant performance. In certain commercially significant acrylamide copolymers, such as copolymers of acrylamide and cationic ester monomers, it is difficult to obtain less than 100 ppm residual acrylamide using either of these methods, even if a longer polymerization time is combined with increased amounts of redox and/or azo initiators.
A photopolymerization process as described, for example, in U.S. Pat. No. 4,178,221, has the potential to produce gelled polymers having very low residual acrylamide content. Such a process has a significant shortcoming, however, relative to a redox/azo system in that the thickness of the gel is limited by the extent to which light can penetrate and initiate polymerization. This thickness depends not only on the light intensity but also on the amount of light-absorbing photoinitiator or sensitizer which is used. The use of less photoinitiator allows light to penetrate to greater depths, but because fewer radicals are produced, the overall rate of polymerization is reduced. For a given light intensity, which is limited by the commercially available equipment, it is therefore necessary to strike a balance between the level of initiator and the gel thickness to achieve a reasonable production rate while still obtaining low residual monomer content in the polymer. This thickness is generally far less than what is practiced using a redox/azo system, so that the production rate for a photopolymerization process is far less for a given factory size.
The combined use of redox initiators and photoinitiator is described in Japanese Patent Publications Nos. JP-P 57-121009 and JP-P 59-133212. Here, the first and greater part of the polymerization is affected by redox initiation, while the last part of the polymerization is affected by light irradiation in combination with the photoinitiator. Alternatively, redox and photopolymerization can occur together as described in German Patent No. 19748153. However, the processes described in these patents do not overcome the aforementioned shortcoming of photopolymerization systems. In particular, the gel thickness is limited by the ability of light to penetrate. For either a photopolymerization process or a combined redox and photopolymerization process, the prior art describes a sequential process of polymerization to produce a gel, during which time irradiation may occur in the case of a photopolymerization or a combined redox and photopolymerization process, followed by comminution of the gel to produce gel particles, followed by drying of the gel particles.